Hydrotalcite is a naturally occurring mineral having the formula: EQU Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3 .multidot.4H.sub.2 O
Several other cation pairs forming hydrotalcite-like structure also occur in nature and of particular interest for the purposes of the present invention is takovite, the nickel analog of hydrotalcite having the formula: EQU Ni.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3 .multidot.4H.sub.2 O
Hydrotalcite-like materials or anionic clay minerals have similar structures and have the general formula: EQU [M.sup.II.sub.1-x M.sup.III.sub.x ](OH).sub.2 .multidot..sup.x /.sub.y A.sup.y- .multidot.mH.sub.2 O
where M.sup.II and M.sup.III are divalent and trivalent cations, respectively, and A is an anion. These materials belong to the pyroaurite-sjogrenite class of minerals and their crystal structure has been described in the literature (Allmann, R., Acta Cryst. (1968), B24, 972). They have been widely described in the literature (Cavani et al., Catalysis Today, 11, 173(1991) and references therein). Although, the word hydrotalcite refers specifically to the Mg-Al mineral, in the catalyst literature, it encompasses all the materials in the pyroaurite-sjogrenite class. Hence for the purposes of this invention, the term hydrotalcite will include materials containing nickel or cobalt and aluminum. The most common approach to synthesis of nickel (or cobalt) hydrotalcites is by coprecipitation of the two cations under conditions of supersaturation (U.S. Pat. No. 3,896,053, 3,941,721, 4,105,591, 4,298,766) and references therein. It is well known that hydrotalcites prepared by the above procedures have a hexagonal plate-like crystal habit (Reichle, W. T., Chemtech, 1986, 58). When crystallized at room temperature the crystallites have a diameter of approximately of about 0.01 to 0.1 microns and can be grown to about 1 to 5 microns by hydrothermal treatment. In all cases, the ratio defined by the diameter to the thickness of hexagonal crystals in such synthetic materials of the prior art are in the range of about 5 to about 20. Scanning and transmission electron microscope (TEM) pictures of hydrotalcite with the hexagonal plate-like crystal morphology are shown in FIGS. 1a and 1b, respectively.
The term "hydrotalcite-like" is recognized in the art. It is defined and used in a manner consistent with usage herein in the comprehensive literature survey of the above-referenced Cavani et al article.